| 摘要 |
The purely bound electromagnetic modes of propagation supported by asymmetric waveguide structures comprised of a thin lossy metal film of fini te width on a dielectric substrate and covered by a different dielectric superstrate have been characterized at optical wavelengths. The dispersion of the modes with film thickness and width has been assessed and the effects caused by varying the difference between the superstrate and substrate dielectric constants on the characteristics of the modes have been determined. In general, the modes supported by these structures exhibit some characteristic s that are consistent with those observed for the modes supported by similar symmetric structures and asymmetric slab guides. Like symmetric structures, the higher order modes have a cutoff width below which they are no longer propagated, and some of the modes have a cut-off thickness. Unlike symmetric structures, all modes that have a decreasing attenuation with decreasing fil m thickness have a cutoff thickness. Under certain conditions, an asymmetric structure can support a long-ranging mode having a field distribution that i s suitable to excitation using an end-fire technique. Like asymmetric slab waveguides, the attenuation of the long-ranging mode near cutoff decreases very rapidly, much more so than the attenuation related to the long-ranging mode in a similar symmetric structure. The cutoff thickness of a long-rangin g mode in an asymmetric finite-width structure is larger than the cutoff thickness of the s b mode in a similar asymmetric slab waveguide. This implies that th e long-ranging mode supported by an asymmetric finite-width structure is more sensitive to the asymmetry in the structure compared to the s b mode support ed by a similar slab waveguide. This result is interesting and potentially usef ul in that the propagation of such a mode can be affected by a smaller change in t he dielectric constant of the substrate or superstrate compared with similar sl ab structures.
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